1. Field of the Invention
The present invention relates generally to charging capacitors and, more particularly, to a method and apparatus for charging high voltage capacitors.
2. Related Art
Sudden cardiac arrest has been attributed to over 350,000 deaths each year in the United States, making it one of the country""s leading medical emergencies. Worldwide, sudden cardiac arrest has been attributed to a much larger number of deaths each year. One of the most common and life threatening consequences of a heart attack is the development of a cardiac arrhythmia, commonly referred to as ventricular fibrillation. When in ventricular fibrillation, the heart muscle is unable to pump a sufficient volume of blood to the body and brain. The lack of blood and oxygen to the brain may result in brain damage, paralysis or death to the victim.
The probability of surviving a heart attack or other serious heart arrhythmia depends on the speed with which effective medical treatment is provided. If prompt cardiopulmonary resuscitation is followed by defibrillation within approximately four minutes of the onset of symptoms, the probability of survival can approach or exceed fifty percent. Prompt administration of defibrillation within the first critical minutes is, therefore, considered one of the most important components of emergency medical treatment for preventing death from sudden cardiac arrest.
Cardiac defibrillation is an electric shock that is used to arrest the chaotic cardiac contractions that occur during ventricular fibrillation, and to restore a normal cardiac rhythm. To administer such an electrical shock to the heart, defibrillator pads are placed on the victim""s chest, and an electrical impulse of the proper magnitude and shape is administered to the victim through the pads. While defibrillators have been known for years, they have typically been complicated, making them suitable for use by trained personnel only.
More recently, portable and transportable automatic and semi-automatic external defibrillators (generally, AEDs) for use by first responders have been developed. A portable defibrillator allows proper medical care to be given to a victim earlier than preceding defibrillators, increasing the likelihood of survival. Such portable defibrillators may be brought to or stored in an accessible location at a business, home, aircraft or the like, available for use by first responders. With recent advances in technology, even a minimally trained individual can operate conventional portable defibrillators to aid a victim in the critical first few minutes subsequent to the onset of sudden cardiac arrest.
As noted, effective medical treatment must be administered promptly after the onset of symptoms. One time consuming defibrillator operation is the charging of a high voltage capacitor that provides the energy for producing the electric shock. Unfortunately, conventional AEDs do not efficiently charge the high voltage capacitor, consuming valuable time preparing to provide the therapy. This limits the number of multiple shocks that can be administered to a patient in the minimal time available. What is needed, therefore, is a defibrillator that can charge a high voltage capacitor quickly and efficiently.
The present invention is a system and method for charging a high-voltage capacitor through the application of a current, the magnitude of which has a fixed frequency waveform. During a charging sequence in which the current is applied repeatedly to the capacitor, the duty cycle of the fixed frequency current waveform is controlled dynamically based on the capacitor voltage. Specifically, the rate at which the energy is transferred to the capacitor is modified according to the efficiency at which the energy can be delivered to the capacitor. This increases the speed at which the high voltage capacitor is charged. Alternative or additional significant benefits may be realized depending on the desired application. For example, systems implementing the present invention can provide charge times comparable to conventional systems using smaller components, a lower energy power source, a higher impedance power source or any reasonable combination thereof
Generally, energy is transferred from a power source to the high voltage capacitor via a magnetic element such as an inductor or transformer. For example, a pulsed voltage supply provides voltage pulses having a constant frequency and an adjustable duty cycle to a primary winding of a fly-back transformer. Initially, there is no energy stored in the transformer core. As a result, the duty cycle of the initial voltage pulse is of sufficient duration to accumulate stored energy in the transformer core. As the quantity of energy stored in the transformer core increases, the transformer is controlled to generate a current to charge the capacitor. The magnitude of the current has a fixed frequency and variable duty cycle waveform.
Specifically, immediately subsequent to the initial accumulation of energy in the transformer core, the duty cycle of the current magnitude waveform is substantial. Since the secondary winding is out of phase with the primary winding (a fly-back transformer), the voltage waveform to cause the generation of such a current magnitude waveform has a substantially small duty cycle. Driving the transformer in such a manner maintains stored energy in the transformer core while providing the secondary winding with sufficient time to transfer energy to the capacitor as the secondary winding cannot otherwise do so in a time-efficient manner due to the minimal capacitor voltage. As the capacitor voltage increases, the duty cycle of the current waveform is decreased in response to an increase in the duty cycle of the voltage waveform. This optimizes the energy transfer rate because the speed at which such transfers can occur increases with an increase in capacitor voltage. Thus, as energy is transferred from the transformer core to the capacitor, a concomitant transfer of energy from the power source to the transformer core occurs. This operating mode is referred to herein as a xe2x80x9ccontinuous modexe2x80x9d since this operational mode insures the transformer core continually stores energy.
A number of aspects of the invention are summarized below, along with different embodiments that may be implemented for each of the summarized aspects. It should be understood that the summarized embodiments are not necessarily inclusive or exclusive of each other and may be combined in any manner in connection with the same or different aspects that are non-conflicting and otherwise possible. These disclosed aspects of the invention, which are directed primarily to high performance capacitor charging systems and methodologies, are exemplary aspects only and are also to be considered non-limiting.
In one aspect of the invention a system for charging a high-voltage capacitor through the application of current the magnitude of which has a fixed frequency waveform is disclosed.
In another aspect of the invention, a high-voltage capacitor charging system is disclosed. The system generates current pulses having a fixed frequency magnitude. In this aspect of the invention, during a charging sequence in which the current pulses are repeatedly applied to a capacitor, the duty cycle of the fixed frequency current waveform is controlled dynamically based on the voltage of the high voltage capacitor.
In a further aspect of the invention, a system for charging a high-voltage capacitor is disclosed. In this aspect of the invention, the system includes a flyback transformer and a pulsed voltage supply. The transformer includes a core, a primary winding and a secondary winding. The pulsed voltage supply provides to the primary winding a voltage having a constant frequency, adjustable duty cycle waveform. The initial duty cycle of the voltage waveform is of sufficient duration to accumulate a quantity of stored energy in the transformer core, after which the voltage waveform is continually applied to the primary coil. The duty cycle of the voltage waveform increases from a substantially small value to a substantially large value during the charging sequence in response to an increase in the instantaneous voltage of the high voltage capacitor.
In a still further aspect of the invention, a system for charging a high-voltage capacitor is disclosed. In this aspect of the invention, the system includes a transformer and a pulsed voltage supply. The transformer includes a core, a primary winding and a secondary winding. A capacitor is electrically coupled across the secondary winding. The pulsed voltage supply provides to the primary winding a voltage having a constant frequency, adjustable duty cycle waveform.
The duty cycle of the voltage waveform is modified dynamically such that energy is stored continually in the transformer core as the capacitor is charged. In particular, during individual cycles of a charging sequence, the system transfers energy from the pulsed voltage supply to the transformer core to replace energy transferred previously from the transformer core to the high voltage capacitor.
In a still further aspect of the invention, a capacitor charging system is disclosed. In this aspect of the invention, the system includes a capacitor charger connected to a capacitor and a diode electrically connected to and interposed between the capacitor and the capacitor charger. The diode has a cathode connected to the capacitor and an anode connected to the capacitor charger. The capacitor charger charges the capacitor by generating a current the magnitude of which has a fixed frequency, variable duty cycle waveform.
In one embodiment of this aspect of the invention, the capacitor charging system includes a magnetic element across which the capacitor is connected, and a pulsed voltage supply connected to a node of the magnetic element, with the other node of the magnetic element connected to ground. The pulsed voltage supply provides to the primary winding a charging voltage that transitions between a first voltage and second voltage that is less than the first voltage at a substantially constant frequency and with a variable duty cycle.
The magnetic element may be a fly-back transformer. In such implementations, the transformer includes a core, a primary winding and a secondary winding that is out of phase with the primary winding. Here, the capacitor is connected across the secondary winding. In one particular implementation, the capacitor charger includes a current sensor connected in series between the other primary winding node and ground. The current sensor generates a voltage having a magnitude that is indicative of current flowing through the primary winding. The charger also includes a control circuit that is operationally coupled to the pulsed voltage supply and the current sensor. The control circuit provides a duty cycle adjust signal to the pulsed voltage supply to adjust the duty cycle of the charging voltage waveform based on the current magnitude signal.
In a further aspect of the invention, a capacitor charger for charging a high voltage capacitor is disclosed. The charger includes a capacitor charging transformer and a charging circuit. The transformer includes a core with primary and secondary windings. The high voltage capacitor is electrically connected across the secondary winding through a diode. The charging circuit is connected to the primary winding and applies a voltage across the primary winding to cause a current to flow through the secondary winding such that the transformer continually stores energy in its core. The secondary winding current transfers energy from the transformer core to the high voltage capacitor.
In another aspect of the invention, a method for charging a capacitor is disclosed. The method includes providing to the capacitor a current the magnitude of which has a fixed frequency waveform. In one embodiment, the duty cycle of the fixed frequency current waveform is varied. The method may also include the steps of: driving a primary winding of a transformer with a fixed frequency, variable duty cycle voltage waveform; sensing an electric current flowing through the primary winding; and adjusting the duty cycle of the voltage waveform when the electric current flowing in the primary winding reaches a predetermined value.
Various embodiments of the present invention provide certain advantages and overcome certain drawbacks of the conventional techniques. Not all embodiments of the invention share the same advantages and those that do may not share them under all circumstances. This being said, the present invention provides numerous advantages including the noted advantage of rapidly transferring energy to a high-energy capacitor. Adjusting the duty cycle of fixed frequency current pulses applied to a capacitor enables energy to be transferred quickly to the capacitor as compared with conventional techniques. In addition, the present invention eliminates the need to sense when the secondary winding of the capacitor-charging transformer has transferred substantially all of its energy to the capacitor. The present invention also eliminates the need to have complex feedback circuitry for adjusting the current in the primary winding of the transformer based on a sensor input from the secondary winding of the transformer. These and other features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.